There is a need in the marketplace to replace coated facestock with uncoated facestock for use in adhesive-backed labels. A driving force behind this need is to cut the high cost associated with the purchase, application, and handling of the coatings that are used to coat such facestock materials.
Requirements that must be met by an uncoated facestock before it can be used to replace certain coated facestocks include obtaining a surface having good flexographic printing, adequate UV adhesive holdout, and minimum edge wick.
In order to attain the micro-smoothness required for good flexographic plate contact and ink transfer, conventional papermaking processes tend to compress the sheet, sacrificing bulk, stiffness and converting performance. Since facestock generally requires only one side to be finished (e.g. for subsequent printing and/or over-laminating), a process which could preferentially treat one surface would be desirable.
Temperature gradient calendering is a known process where the surface of the paper is heated to a temperature higher than the glass transition temperature of the cellulose in the nip while the bulk temperature of the sheet is substantially cooler. This process enables smoothness development with reduced bulk loss compared to conventional hard-nip (machine calendering) or hot-soft calendering. In addition, surface moisturization can also be used to lower the glass transition temperature preferentially closer to the surface to develop smoothness with minimum bulk loss.
Hot-soft calendering, another method of calendering that is commonly used for coated substrates, also relies on the temperature gradient calendering concept, but the web that is being pressed against a hot surface in a nip is supported by a roll that has a deformable cover. This cover gives the paper a longer dwell time in the nip compared to hard nip calendering and also allows the smoothness and gloss development to occur at relatively uniform density across the width of the paper. Soft calendering is an expensive option for existing machines and has limitations, such as cover delamination and cracking due to overheating.
Extended nip calendering extends the soft calendering concept to longer nip widths and reduces the operational problems. One type of extended nip calendering uses an endless band/belt over a backing roll to provide support for a paper web that is pressed against a heated cylinder. Another variation to this concept is to use a shoe instead of a roll as a backing for the paper web. The backing shoe provides longer nip widths, hence an increase in dwell time and a decrease in peak nip pressure. Use of extended nip calendering has provided substantial benefits for the manufacture of certain paper products such as envelope paper (reference U.S. Pat. No. 6,332,953).
Based on the high cost of coating conventional coated facestock, there is clearly a need in the market for uncoated facestock materials that have suitable or improved performance in comparison to conventional coated facestock.